1. Field of the Invention
This invention relates to valves, and more particularly, to pressure ratio valves for generating a pneumatic control signal for controlling an air actuatable throttle control means in response to the ratio of the pressures of a fluid and a high pressure air supply.
2. Description of the Prior Art
In the operation of internal combustion engines it is frequently desirable to provide throttle control means for automatically limiting the engine RPM when certain conditions occur which would be detrimental to the engine. To provide such protection, throttle control means such as an air actuatable throttle control cylinder have been designed to forcefully and automatically reduce the engine RPM to a predetermined value, such as idle RPM, in response to a change in a pneumatic control signal. A separate pressure ratio valve senses the pressure ratio between the vehicle's oil pressure and its compressed air system pressure. The combination of the throttle control cylinder and the pressure ratio valve prevents the occurrence of high engine RPMs during starting before the engine oil system has attained its normal operating pressure. The system also automatically and forcefully reduces the engine RPM to a predetermined level whenever the engine oil pressure-air pressure ratio decreases below a predetermined value. This latter event would occur if the oil pressure system became obstructed or an oil line ruptured causing a dangerously low oil pressure.
One prior art pressure ratio valve which accomplishes the above-mentioned objectives is manufactured by Sentinel Distributors, Inc., and is structurally identical to the fuel shut-off device disclosed in U.S. Pat. No. 3,523,521. This apparatus is exceedingly sophisticated, complex, and expensive. It utilizes a pressure sensing piston in combination with a ball and seat valve and a pair of biasing springs to bias the ball and seat valve and the piston into predetermined positions. The ball and seat valve is extremely susceptible to damage from foreign particulate matter circulated within the compressed air system of a vehicle. The Sentinel device also includes a dial-operated, cam driven override assembly disposed in the lower portion thereof to open the ball and seat valve in order to permit engine operation under certain conditions. The Sentinel device is extremely difficult to manufacture and assemble since four radially inwardly extending annular seats must be fabricated within its longitudinally extending bore to provide proper seating and support for the various internal elements. Furthermore, the Sentinel device requires two removable end caps so that the various internal components can be inserted from both ends during assembly. As installed in an internal combustion engine system, the Sentinel device requires a special vented valve assembly in series with a quick release valve between the control port and the input to the throttle control cylinder. The installation of this device is thereby greatly complicated and overall system reliability is thereby decreased.